1. Field of the Invention
The present invention relates to an improvement of a scanning optical system for use in a laser beam printer and other devices that employ a semiconductor laser as a light source.
2. Background of the Invention
The basic components of a scanning optical system in a laser beam printer are a light source that emits a light beam, a deflector for deflecting the emitted light beam and a scanning lens unit for concentrating the deflected beam at a position corresponding to the angle of deflection. In most cases, the light source is in the form of a small-sized and directly turnable semiconductor laser. Since a semiconductor laser emits divergent light, it is customarily used in combination with a collimator lens that is capable of producing a beam of parallel rays from the divergent light. The angle of divergence of light emitted from the semiconductor laser differs between a direction parallel to the plane of junction in the laser (this direction is hereinafter referred to as the parallel direction) and a direction perpendicular to the junction plane (this direction is hereinafter referred to as the perpendicular direction). The divergence in the perpendicular direction has a larger angle than in the parallel direction. As a result, the beam that has passed through a collimator lens has a larger diameter in the perpendicular direction than in the parallel direction and the beam that is finally concentrated on the scanning surface by the scanning lens unit has a smaller F number in the perpendicular direction than in the parallel direction. In other words, the spot diameter which is proportional to the F number of the concentrated beam is larger in the parallel direction than in the perpendicular direction.
In the prior art, various techniques have been employed to solve this problem. One is to reduce the effective aperture of the collimator lens at the expense of energy efficiency so as to restrict the beam in the perpendicular direction, thereby producing a substantially circular beam. Another approach is to effect beam shaping with an anamorphic optical device such as a prism.
The conventional scanning system has another disadvantage in that deflectors such as a rotating polygonal mirror is prone to the tilting of deflecting planes, which causes an error not only in the direction in which light scanning is effected (this direction is hereinafter referred to as the main scanning direction) but also in the direction perpendicular to that scanning direction (this direction is hereinafter referred to as the sub-scanning direction). This error will lead to uneveness in the pitch between scanning lines.
Various techniques have been proposed for compensating for the tilting of deflecting planes. In one method, an anamorphic optical unit is disposed in front of the deflector so that laser beam will be focused to form an image on the plane of deflection in a cross section as taken by cutting the scanning optical system in the sub-scanning direction. At the same time, an anamorphic scanning lens system is used to focus the laser light to form another image on the scanning plane so that the scanning plane and the plane of deflection will provide two conjugate planes to thereby eliminate any adverse effect of the tilting of deflecting planes. In another method, an anamorphic optical unit and an anamorphic scanning lens system are also used to reduce the focal length and the magnification of image in the sub-scanning direction sufficiently to reduce the adverse effects of the tilting of deflecting planes. However, the former method in which laser light is focused to form a linear image on the plane of deflection has the disadvantage that it is vulnerable to surface flaws or dust buildup on the plane of deflection. In addition, the image formed is highly sensitive to a change of the deflection point of the polygonal mirror. In the latter method, a complicated optical system is required to effect beam shaping and only insufficient compensation of the tilting of deflecting planes can be achieved.
The scanning lens unit used to concentrate a deflected beam on the scanning plane at a position corresponding to the angle of deflection is usually composed of an f-.theta. lens which is capable of attaining a proportional relationship between the angle of incidence and the height of image. However, in order to attain the proper proportionality between incident angle and image height (this relationship is hereinafter referred to as linearity), this f-.theta. lens has a strong negative distortion but this distortion is determinative to the purpose of reducing the error in the linearity between incident angle and image height to a very low level.